The invention lies in the field of treating waste materials to recover zinc and other metal values.
Because of the nature of the operation of electric steel furnaces, large amounts of scrap material can be fed into the steel-making process, which was previously impossible, and this is responsible for the large amount of zinc contained in the flue dust from these furnaces. Due to the high proportion of zinc in the dust, it is especially desirable to further refine or treat the dust to remove zinc values from this dust for sale. Naturally, if this dust can be recovered in a simple and relatively inexpensive manner, the commercial value of the zinc obtained would help defray the cost of making steel, and some gain could be recovered from formerly "waste" material.
As is well known in the art, inherent in the process for making steel by use of electric steel furnaces with submerged arcs is the liberation of zinc, iron, and other metal values as flue dust. This "flue dust" is usually relatively high in zinc and iron, and also contains small amounts of copper, cadmium, lead, sulfur, chromium and other metals. Therefore, it is a primary purpose of this invention to recover metal values from this steel-making flue dust, and particularly zinc values, and additionally to provide a means for the separation and recovery of other materials in the dust should this be desirable.
U.S. Pat. No. 2,805,918 to Van Hare, et al., discloses a method for leaching zinc-copper mixtures with an ammoniacal-ammonium carbonate liquor to solubilize the zinc and copper for subsequent separation. Although this patent teaches a method for removing zinc and copper from solution, this process would not be satisfactory for use with flue dust as the initial feed material, since flue dust does not contain only zinc and copper values, but a multitude of other metallic values which must also be removed to provide an essentially pure zinc product. The method disclosed in the patent for removal of zinc is precipitation of the zinc by heating; however, with amounts of lead, nickel, sulfur, chromium, cadmium, and some iron in solution, these values would also be precipitated by heat with a resultant residue of little value because of its impurity. H. M. Lawrence, in Bull. Am. Zinc. Inst., 10, No. 5-6, 107-18 (1927), discloses a process for first leaching a zinc ore with an ammonium carbonate leach, and then cementation with zinc to replace some impurities on the metal sites of the complex with zinc and precipitate impurities; however, Lawrence also was not working with flue dust, which usually contains amounts of iron, chromium and sulfur high enough to render a final product treated by this method hopelessly impure.
"Transactions AIME," Journal of Metals (Vol. 180, January 1950), p. 85, discloses a method for steam distilling a pregnant nickel carbonate liquor to remove impurities, including iron, and produce a basic nickel carbonate product, and this procedure, coupled with the product of Lawrence's cementation step, would be a considerable development were nickel and zinc equivalent metals. Obviously, however, one skilled in the art would not equate nickel and zinc in most instances; and even if these references were combined and used to treat flue dust, substantial amounts of sulfur and chromium would still be left in the basic zinc carbonate product of the steam distillation step.
It is obvious, then, from the above information and from the state of the prior art that a method is needed for separating and purifying zinc from steel-making flue dust which can economically remove all of the metallic impurities commonly found in flue dust, as well as result in a substantially pure zinc product.